Optical signal-electric signal converter

ABSTRACT

An optical signal to electrical signal converter of the present invention comprises an optical waveguide propagating a modulated optical signal therethrough; a pair of electrodes disposed at positions opposite to each other sandwiching the optical waveguide with in a region where an electric field reaches that is generated in the optical waveguide when the optical signal propagates through the optical waveguide; and a resonator coupled to the pair of electrodes, the resonator receiving for excitation an electrical signal induced at the pair of electrodes by the electric field.

TECHNICAL FIELD

The present invention relates to an optical signal to electrical signalconverter using a nonlinear optical effect.

BACKGROUND ART

Conventionally, as a device for converting an optical signal to anelectrical signal there have been widely used an electronic tuberepresented by a photo-multiplier and a semiconductor photodetectorrepresented by a photodiode. The electronic tube is a device whichdetects an optical signal utilizing the “external photo-electric effect”and the semiconductor photodetector detects an optical signal utilizingthe “internal photo-electric effect” in the semiconductor.

The electronic tube has high detection sensitivity (signal amplificationfactor) and is often used still currently in uses for physics andchemistry, however, is large and needs a high-voltage power source foroperation. Therefore, the electric tube is mostly not used in uses as aphotodetector for optical communications.

In contrast, because the semiconductor photodetector is small andconsumes a little electric power, it is used in a wide range of fieldsincluding the optical communication field. Among semiconductorphotodetectors, pin-type photodiodes (pin-PD) are inexpensive and areused for various uses. However, avalanche photodiodes (APD) capable ofresponding at a high speed are used for high-speed opticalcommunications. In recent years, pin-type photodiodes with an improvedresponse speed have been also developed. Then, the current situation issuch that these types of semiconductor photodetectors can be used withsubstantially no problem at the current communication speeds (bandwidth<60 GHz band).

However, there is a problem that the semiconductor photodetector can notrespond sufficiently in an ultra-high frequency band where thecommunication speed exceeds 100 GHz. This is because the response speedof the semiconductor photodetector is limited by the mobility ofelectric carriers generated by the input of an optical signal.

For the pin-type photodiode, pairs of an electron and a hole aregenerated when light is incident on a light-absorbing layer of thephotodiode. The mobility of the hole is smaller than that of theelectron. The delay time that determines the response speed of thephotodiode is limited by the drift speed of holes. In this manner, theresponse speed of the semiconductor photodetector is determined byfactors including the carrier mobility inherent to the semiconductormaterial, the voltage applied and the drift length. However, even whenthese parameters are further increased, it is considered difficult toimprove the response speed up to a response speed with which an opticalsignal modulated at a speed exceeding 100 GHz can be accuratelydetected.

The present invention was conceived in order to solve the above problemand the primary objective thereof is to provide an optical signal toelectrical signal converter capable of converting an optical signalmodulated at a high speed into an electrical signal.

DISCLOSURE OF THE INVENTION

An optical signal to electrical signal converter of the presentinvention comprises an optical waveguide for receiving and propagating amodulated optical signal; and a pair of electrodes disposed at positionsopposite to each other sandwiching the optical waveguide within a regionwhere an electric field applies, said electric field being generated inthe optical waveguide by a nonlinear optical effect when the opticalsignal propagates through the optical waveguide.

In a preferred embodiment, the optical signal to electrical signalconverter further comprises a resonator coupled to the pair ofelectrodes. The resonator is capable of be excited by an electricalsignal induced at the pair of electrodes by the electric field.

In a preferred embodiment, the optical signal contains a side bandsignal corresponding to a modulation frequency f_(m).

In a preferred embodiment, the optical waveguide is formed on adielectric substrate or in the dielectric substrate, with the electrodesbeing supported by the dielectric substrate.

In a preferred embodiment, at least a portion of the optical waveguideand at least a portion of the dielectric substrate are formed from anonlinear optical material and generate the electric field by an opticalrectifying effect when the optical signal propagates through the opticalwaveguide.

In a preferred embodiment, the optical signal to electrical signalconverter further comprises an electromagnetic wave radiating devicecoupled to the resonator and radiates the electrical signal as a radiosignal.

In a preferred embodiment, the resonator and the electromagneticradiating device are integrated with the substrate.

In a preferred embodiment, the resonator and the electrodes areconnected by micro strip lines formed on the dielectric substrate.

In a preferred embodiment, the modulation frequency of the opticalsignal is 10 GHz or higher.

In a preferred embodiment, the optical signal to electrical signalconverter further comprises a light beam input portion coupled to theoptical waveguide.

In a preferred embodiment, the nonlinear optical material is a materialselected from a group consisting of lithium niobate (LiNbO₃), lithiumtantalate (LiTaO₃)-based material, potassium titanyl phosphate(KTiOPO₄)-based material, rare earth-calcium oxyborate (RECa₄O(BO₃)₃,RE: a Rare Earth element)-based material, DAST(4-dimethylamino-N-methyl-4-stilbazorium-toxyrate) and 3RDCVXY(dicyanovinyl termination-dimethyl substitution-diazo).

In a preferred embodiment, the optical waveguide has a periodicpolarization inversion structure where the polarization direction isdifferent from the polarization direction in the other portion.

In a preferred embodiment, the optical signal to electrical signalconverter further comprises a resistor connecting electrically the pairof electrodes with each other.

In a preferred embodiment, the optical signal to electrical signalconverter further comprises a housing accommodating the dielectricsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an embodiment of an optical signalto electrical signal converter according to the present invention;

FIG. 2 is a perspective view showing another embodiment of the opticalsignal to electrical signal converter according to the presentinvention;

FIG. 3 is a perspective view showing a yet another embodiment of theoptical signal to electrical signal converter according to the presentinvention;

FIG. 4(a) is a perspective view showing the main portion of the opticalsignal to electrical signal converter of FIG. 3, with a resonatorremoved therefrom; FIG. 4(b) is a cross-sectional view of the mainportion taken along the line A-A′ of FIG. 4(a); and FIG. 4(c) is across-sectional view of the main portion taken along the line B-B′ ofFIG. 4(a);

FIG. 5(a) is a perspective view of a dielectric resonator antenna of theoptical signal to electrical signal converter of FIG. 3; and FIG. 5(b)is a cross-sectional view of the dielectric resonator antenna takenalong the line A-A′ of FIG. 5(a); and

FIG. 6 is a perspective view showing connections between electrodes andthe dielectric resonator in the converter of FIG. 3.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, an optical signal is converted into anelectrical signal without utilizing drifts of carriers excited byincident light, but by utilizing a nonlinear optical effect. Therefore,the response speed is not limited by the drift speed of the majority ofcarriers.

In the following, the principle of the operation of the optical signalto electrical signal converter according to the present invention willbe described.

The polarization of a material having a nonlinear optical effect isrepresented by the following Eq. 1.D=∈E+P _(NL)  Eq. 1where D is an electric displacement vector (the electric flux density),∈ is a dielectric constant, E is an electric field and P_(NL) isnonlinear polarization.

As represented by Eq. 1, the electric displacement vector D is normallya sum of the nonlinear polarization P_(NL) and the product of thedielectric constant ∈ and the electric field E. The term for thenonlinear polarization P_(NL) can be represented by the following Eq. 2taking into consideration only the quadratic nonlinear optical effect.P _(NL)=χ⁽²⁾ E·E  Eq. 2where χ⁽¹⁾ is the quadratic nonlinear polarizability.

The light incident on the nonlinear optical material is assumed to berepresented as the sum of two (2) electric fields E₁ and E₂ representedin the following Eq. 3.E ₁ =E ₀₁ cos(ω₁ t−κ ₁ r+φ₁),E ₂ =E ₀₂ cos(ω₂ t−κ ₂ r+φ ₂)  Eq. 3where ω₁ and ω₂ are frequencies of light beams, t is the time, κ₁ and κ₂are wave numbers of the light and φ₁ and φ₂ are phases of the light.

Using Eq. 3, the square of the electric field E in Eq. 2 is representedas follows.E·E=(E ₁ +E ₂)·(E ₁ +E ₂)=E ₀₁ ² cos²(ω₁ t−κ ₁ r+φ ₁)+2E ₀₁ E ₀₂ cos(ω₁ t−κ ₁ r+φ ₁)·cos(ω₂ t−κ ₂r+φ ₂)+E ₀₂ ² cos²(ω₂ t−κ ₂ r+φ ₂)  Eq. 4

Using the relationship of cos 2θ=2 cos²θ−1, Eq. 4 is represented as“Term A+Term B+Term C+Term D+Term E” in the following Eq. 5.E·E=½(E ₀₁ ² +E ₀₂ ²) (Term A)+½E ₀₁ ² cos(2ω₁ t−2κ₁ r+2φ₁) (Term B)+½E ₀₂ ² cos(2ω₂ t−2κ₂ r+2φ₂) (Term C)+E ₀₁ E ₀₂ cos[(ω₁+ω₂)t−(κ₁+κ₂)r+(φ₁+φ₂)] (Term D)+E ₀₁ E ₀₂ cos[(ω₁−ω₂)t−(κ₁−κ₂)r+(φ₁−φ₂)] (Term E)  Eq. 5

Term A of Eq. 5 is a term for optical rectification. Term B and Term Crepresent generation of the secondary harmonics, Term D representsgeneration of the summed frequency, and Term E represents generation ofthe differential frequency.

According to the present invention, using the effect represented by TermE among the nonlinear optical effects represented in Eq. 5, an opticalsignal is converted into an electrical signal. This point will bedescribed in detail as follows.

For a light beam (having the central frequency of 1.5 μm) modulated witha signal having the central frequency of, for example, 26 GHz band by anoptical modulating device, a peak called “side band” is generated at aposition 0.19 nm away from the central frequency. In general,representing the frequency of the modulating signal as f_(m) Hz, thefrequency λ_(sb) at which the side band is generated is represented asthe following Eq. 6.λ_(sb)=λ_(c)+ΔλΔλ=λ_(c) −Cλ _(c)/(C+f _(m)π)=f _(m)π_(c) ²/(C+f _(m)λ_(c))  Eq. 6

-   -   C: the light velocity    -   λ_(c): the central frequency of the light beam    -   f_(m): the frequency of the modulating signal

The optical signal to electrical signal converter of the presentinvention carries out the conversion into a modulated signal by thegeneration of the differential frequency (Eq. 7) between the wavelengthλ_(sb) of this side band and the central wavelength λ_(c).ω_(m)=ω_(sb)=ω_(c)

That is,1/λ_(m)=1/λ_(sb)−1/λ_(c) =f _(m) /C  Eq. 7

-   -   ω_(m): the angular frequency of the modulating signal    -   ω_(sb): the angular frequency of the side band    -   ω_(c): the angular frequency of the central frequency    -   ω_(m): the wavelength of the modulating signal    -   ω_(sb): the wavelength of the side band    -   ω_(c): the central wavelength

Here, for convenience, the case where two light beams each having afrequency (wavelength) different from each other are input into anonlinear optical material is considered. However, the case where onelight beam having one frequency (wavelength) is input may be consideredsimilarly to the above case.

Now, a preferable embodiment of the optical signal to electrical signalconverter according to the present invention will be described.

First Embodiment

First, referring to FIG. 1, the configuration of the optical signal toelectrical signal converter of the embodiment will be described.

The optical signal to electrical signal converter of the embodiment hasa dielectric substrate 101 formed from a nonlinear optical material, anoptical waveguide 102 formed on the upper face of the substrate 101 anda pair of electrodes 103 and 104 provided at positions opposite to eachother sandwiching the optical waveguide 101 on the upper face of thesubstrate 101.

An optical signal to be detected is incident on the input portion 102 aof the optical waveguide 102 and propagates through the opticalwaveguide 102 from the left to the right in the figure. At this time, anelectric field is generated by the differential frequency generationeffect among the nonlinear optical effects. The pair of electrodes 103and 104 is provided within a region where the electric field generatedin the optical waveguide 102 reaches.

According to the composition of the embodiment, variation of theelectric field generated when the optical signal propagates through theoptical waveguide 102 from the left to the right in the figure can bedetected through the electrodes 103 and 104. As has been described, thiselectric field is formed in the optical waveguide and in the vicinity ofthe optical waveguide by the differential frequency generation of thenonlinear optical effect. In order to convert the optical signal into anelectrical signal by generating such a differential frequency, theoptical signal inputted needs to be a signal modulated such that theoptical signal has a side band signal.

In the embodiment, lithium niobate (LiNbO₃) substrate may be preferablyused as the dielectric substrate 101. The material of the substrate 101is not limited to lithium niobate (LiNbO₃), and lithium tantalate(LiTaO₃), potassium titanyl phosphate (KTiOPO₄), rare earth-calciumoxyborate (RECa₄O(BO₃)₃, RE: a Rare Earth element), DAST(4-dimethylamino-N-methyl-4-stilbazorium-toxyrate) or 3RDCVXY(dicyanovinyl termination-dimethyl substitution-diazo) may also be used.

Next, a method for manufacturing the optical signal to electrical signalconverter shown in FIG. 1 will be described.

First, ultrasonic cleaning is applied to the substrate 101 in a liquidsuch as distilled water, acetone or alcohol. Thereafter, ultrasoniccleaning is applied to the substrate 101 also in acetic acid for oneminutes or less. Again, ultrasonic cleaning is applied to the substrate101 in a liquid such as distilled water, acetone or alcohol.

Next, a resist mask for defining the position and the shape of theoptical waveguide 102 is formed on the upper face of the substrate 101using a photo-lithography method. Thereafter, a Ti film is deposited onthe resist mask using an electron beam deposition method. The thicknessof the Ti film is set at, for example, 40-50 nm.

Next, the portion of the Ti film except the area where the opticalwaveguide 102 is to be formed on is removed using a lift-off method. Inthis manner, a Ti film patterned to define the area where the opticalwaveguide is formed. The method to form the Ti film is not limited tothe electron beam deposition method, and sputtering such as the RFmagnetron sputtering method may be used.

Next, the substrate 102 on which the patterned Ti film is present on thesurface is loaded into a tube furnace and Ti is diffused in the surfaceregion of the substrate 102. The tube furnace has a heater and a quartztube heated by this heater. The substrate 101 is set on a quartz boatplaced in the quartz tube. As the ambient gas in the quartz tube, Ar gascontaining steam and having the humidity of 80% or more is used for thefirst five hours of the diffusion process. After the first five hours,the ambient gas is switched to O₂ gas containing steam and having thehumidity of 80% or more and the substrate 101 is heated for around onehour. The temperature for heating is set at, for example, around 1,000°C. The reason why the substrate 101 is heated in the O₂ atmosphere forthe last one hour of the Ti diffusion process is in order to compensatethe oxygen defects generated in the substrate 101.

In this manner, the optical waveguide 102 is formed on the substrate101. The method for forming the optical waveguide 102 is not limited tothe Ti diffusion method, and methods in which transition metals such asV (vanadium), Ni (nickel) and Cu (cupper) are respectively diffused maybe used. Otherwise, a method in which protons exchange is carried out bydipping the substrate 101 in the melted salt of benzoic acid for around24 hour may be employed.

When an organic nonlinear optical material such as DAST(4-dimethylamino-N-methyl-4-stilbazorium-toxyrate) or 3RDCVXY(dicyanovinyl termination-dimethyl substitution-diazo) is used for thesubstrate 101, it is preferable to form the optical waveguide using arefractive index variation method (photo-bleaching method) employingillumination of a UV light beam.

The width and the depth of the optical waveguide 102 are both around 5μm in this embodiment. However, the width and the depth of the opticalwaveguide 102 are optimized with the wavelength of the optical signalsto be guided.

Next, the electrodes 103 and 104 extending along the optical waveguide102 are formed. More specifically, first, an aluminum film is depositedon the upper face of the substrate 101 formed with the optical waveguide102, using the electron beam deposition method. The material for theelectrode is not limited to aluminum, and simple substances or alloys ofplatinum, gold, titanium, germanium etc. may be used. After depositingthe metal film or a film of another conductive material, the electrodes103 and 104 can be formed by patterning the conductive film usingvarious methods. The patterning of the electrodes 103 and 104 may becarried out using the lift off method.

It is preferable to form a thin film made of SiO₂, HfO₂ or SiN, thatworks as a protective film, over the whole area of the upper face of thesubstrate 101 before forming the electrodes 103 and 104.

Next, a terminal resistor 105 (50 Ω) is connected respectively with oneend of each of the electrodes 103 and 104 such that the terminalresistor 105 bridges the electrodes 103 and 104 and the converter shownin FIG. 1 is completed. Relaxation of the difference in the phasevelocity between an optical signal and an electrical signal can beachieved by the terminal resistor 105 as the electrodes of atraveling-wave-type optical modulator.

The effective nonlinear optical constant d_(eff) of a nonlinear opticalmaterial is proportional quadratically to the power of a generatedelectrical signal as represented in Eq. 8.P=Ad _(eff) ² L ² P ₁ P ₂[(sin x)/x] ² /n ₁ n ₂ n ₃λ₃  Eq. 8

-   -   A: proportional constant    -   d_(eff): nonlinear optical constant    -   L: the length of a crystal    -   P₁: the power of an input light beam 1    -   P₂: the power of an input light beam 2    -   n₁: refractive index to the input light beam 1    -   n₂: refractive index to the input light beam 2    -   n₃: refractive index to an output (a square of the dielectric        constant)    -   λ₃: the wavelength of the output

The above input light beam 1 is a signal at the central frequency of anoptical signal inputted into the waveguide and the input light beam 2 isa signal of the side band.

From the above, it is preferable to form the optical waveguide using amaterial having a high effective nonlinear optical constant d_(eff).Generally, organic nonlinear optical materials have higher effectivenonlinear optical constants d_(eff) than inorganic nonlinear opticalmaterials. Therefore, the detection sensitivity for optical signals ismore improved and conversion efficiency from an optical signal into anelectrical signal is more enhanced when an organic nonlinear opticalmaterial is used. The effective nonlinear optical constant d_(eff) ofLiNbO₃ crystal that is one of the inorganic crystal having a relativelyhigh effective nonlinear optical constant is around 30 pm/V. Incontrast, the effective nonlinear optical constant d_(eff) of DASTcrystal that is one of organic crystals is 1,000 pm/V that is a highvalue of 30 times as high as or higher than the effective nonlinearoptical constant d_(eff) of LiNbO₃. Therefore, DAST is preferably usedas the material for the substrate or the optical waveguide of theembodiment.

An effective nonlinear optical constant d_(eff) varies depending on thedirection of incidence of a light beam. Therefore, the direction ofincidence of a light beam is preferably in the x-y plane when alithium-niobate-based or a lithium-tantalate-based crystal is used.

For not only the niobate-based nonlinear optical crystal but alsoeffective nonlinear optical crystals, each of the equations representingthe incident angle of a light beam to a crystal and the effectivenonlinear optical constant d_(eff), using the crystal system (the pointgroup and the space group) that the crystal has is different between thetwo kinds of crystals. Therefore, it is necessary to select an angle atwhich the effective nonlinear optical constant d_(eff) becomes maximalaccording to the kind of the crystal.

For example, for a LiNbO₃ crystal, because the crystal is a uniaxialcrystal and has a point group 32, Eq. 9 representing the crystallineangle and the effective nonlinear optical constant is represented asfollows.d _(eff) =d ₁₁ cos 2θsin 3φ  Eq. 9where θ is the angle formed by the z-axis and a component projected ontothe x-z plane of the dielectric principal axis in the incident directionof the light beam, and φ is the angle formed by the x-axis and acomponent projected onto the x-z plane of the dielectric principal axisin the incident direction of the light beam.

The optical signal to electrical signal converter according to thepresent invention can further reduce the difference in the phasevelocity between an optical signal and an electrical signal byintroducing a polarization inversion structure into the opticalwaveguide section. The optical signal to electrical signal converter canalso carry out pseudo matching of velocities by introducing thepolarization inversion structure. Then, a higher effective nonlinearoptical constant d_(eff) can be obtained than the case where matching ofvelocities is carried out by the incident angle of the light beam intothe crystal. The sensitivity can be improved and the conversionefficiency from an optical signal to an electrical signal can be madehigher by introducing the polarization inversion structure.

Second Embodiment

Next, referring to FIG. 2, a second embodiment equipped with an opticalwaveguide having a structure in which polarization is periodicallyinverted will be described.

The basic structure of the embodiment is almost same as the structure ofthe converter shown in FIG. 1 except an optical waveguide 203 having apolarization inversion structure 202. That is, an optical signal toelectrical signal converter of the embodiment has a substrate 201, theoptical waveguide 203 formed on the upper face of the substrate 201 anda pair of electrodes 204 and 205 provided at positions opposite to eachother on the upper face of the substrate 201. When an optical signalincident upon the input portion 203 a which is located at an end of theoptical waveguide 203 propagates through the optical waveguide 203 fromthe left to the right in the figure, an electric field is generated bythe differential frequency generation effect among the nonlinear opticaleffects. The pair of electrodes 204 and 205 is provided within a regionwhere the electric field generated in the optical waveguide 202 reaches.One end of each of the electrodes 204 and 205 is connected with eachother by a terminal resistor 206.

The material, the size and the manufacturing method of this converterare basically same as those described for the first embodiment. Becausethe different point of the embodiment from the first embodiment is thatthe polarization inversion structure 202 is manufactured, this pointwill be described as follows.

In the embodiment, first, metal electrodes are deposited on thesubstrate 201 using the electron beam deposition method. Morespecifically, comb-type electrodes are formed on the upper face of thesubstrate 201 and a front-face electrode is formed on the back face ofthe substrate 201. As the material for the metal electrodes, simplesubstances or alloys of aluminum, platinum, gold, titanium, germanium,nickel or etc. is preferably used. The comb-type electrodes aremanufactured by patterning the electrodes with photolithography andetching techniques after depositing the metal film on the substrate 201.However, the comb-type electrodes may be formed using the lift offmethod after forming a patterned resist mask on the substrate anddepositing a metal film.

After the comb-type electrodes have been completed, the direction of thepolarization in a specific area of the optical wave guide 202 isinverted against the direction of the polarization in the other area byforming an electric field between the electrodes on the upper face ofthe substrate 201 and the electrode on the back face of the substrate201.

The polarization inversion period Λ=2L_(c) is calculated from thefollowing Eq. 10 or Eq. 11.L _(c)=λ_(m)/2(n _(g) −n _(m))  Eq. 10L _(c)=½f _(m)(1/v _(m)−1/v _(g))  Eq. 11where L_(c) is the coherence length, n_(g) is the refractive index of alight beam, n_(m) is the refractive index of an electric wave, v_(g) isthe group velocity of the light beam, λ_(m) is the phase velocity of theelectric wave, f_(m) is the wavelength of an electromagnetic wave andf_(m) is the frequency of the electric wave.

In the embodiment, a He—Ne laser beam is used as the optical signal and,because f_(m) is 26 GHz, v_(m) is 6.4×10⁷ m/s, v_(g) is 1.36×10⁸ m/s,the coherence length is 2.4 mm and the polarization inversion period is4.7 mm.

After forming the polarization inversion structure 202, ultrasoniccleaning is applied to the substrate 201 in a liquid such as distilledwater, acetone or alcohol. The manufacturing process thereafter is sameas the process described for the first embodiment.

In each of the above embodiments, the arrangement for detecting anelectrical signal through the electrodes is not limited especially. Theelectrical signal may be amplified using a known high-sensitivitydetection circuit. However, because the electrical signal converted froman optical signal is very weak in the optical signal to electricalsignal converters in the above embodiments, it is preferable to equip amechanism for amplify the very weak signal easily.

Description will be given below of an embodiment in which the electricalsignal converted from an optical signal is amplified using a resonator.

Third Embodiment

Now, a third embodiment of the optical signal to electrical signalconverter according to the present invention will be described. In theembodiment, a polarization inversion structure is introduced into theoptical waveguide section as well as an antenna (electromagnetic waveradiation device) is connected to the electrodes through a dielectricresonator. The periodic polarization inversion structure in theembodiment has the same structure as the structure in the secondembodiment.

Referring to FIG. 3, the converter of this embodiment will be described.

The converter has a substrate 301 accommodated in the housing 309.Similarly to the second embodiment, an optical waveguide 302 having theperiodic polarization inversion structure is formed on the substrate301. A pair of electrodes 303 a and 303 b is formed positions oppositeto each other sandwiching the optical waveguide 302 on the upper face ofthe substrate 301. When an optical signal is incident on the inputportion located at an end of the optical waveguide 302 propagatesthrough the optical waveguide 302 from the left to the right in thefigure, an electric field is generated by the nonlinear optical effect.The pair of electrodes 303 a and 303 b is provided within a region wherethe electric field generated in the optical waveguide 302 reaches.

A dielectric resonator 304 equipped with an electromagnetic waveradiation mechanism 306 is provided on a section located on an opticaloutgoing side 308. A terminal resistor 305 (50 Ω) is formed on anoptical incidence side 307.

Next, referring to FIG. 4(a) to FIG. 4(c), the configuration of theconverter of the embodiment will be described more specifically. FIG.4(a) is a perspective view showing the main portion, in which theresonator is removed from the optical signal to electrical signalconverter of the embodiment. FIG. 4(b) is a cross-sectional view of themain portion along the line A-A′ in FIG. 4(a) and FIG. 4(c) is across-sectional view of the main portion along the line B-B′ in FIG.4(a).

As shown in FIG. 4(b), a polarization inversion structure 403 is formedin the optical waveguide 302 on the substrate 301 accommodated in thehousing 309. The polarization inversion structure 403 is a structure inwhich areas where the polarization direction of the material of thesubstrate is inverted are arranged periodically. The cycle of thearrangement is set at a length equal to the coherent length of anoptical signal input. The optical waveguide 302 in the embodiment isdesigned to propagate an optical signal having the wavelength of 633 nm.

The housing is a case made of metal covering the bottom face and theside faces of the substrate 301 and cutouts are formed in portions eachcorresponding respectively to the sections for the optical signals toenter and go out of the substrate 301. In order to reduce the influenceof the external electromagnetic waves, the housing 309 preferably has ashape such that the upper face of the substrate 301 is covered with acover section (not shown).

As shown in FIG. 4(c), the electrodes 303 (303 a and 303 b) are formedalong the optical waveguide 302 and can detect a very weak electricfield generated in the optical waveguide 302. A portion of each of theelectrodes 303 a and 303 b in the embodiment is buried inside thesubstrate 301 from the upward. However, the electrodes may not have sucha structure. For example, electrodes obtained by patterning a metal filmdeposited on the upper face of the substrate 301 may be used.

Next, referring to FIG. 5(a) and FIG. 5(b), a dielectric resonatorsection and an electromagnetic field radiation mechanism will bedescribed. FIG. 5(a) shows the schematic composition of the dielectricresonator. FIG. 5(b) is a cross-sectional view of the dielectricresonator along the line A-A′ in FIG. 5(a).

A dielectric resonator 304 comprises a metal housing 501 for insulatingthe electromagnetic field inside the resonator from the exterior and ahigh-dielectric-constant dielectric 503 arranged inside the housing 501.A material having a relatively low dielectric constant (for example, amaterial having a commercial name, Teflon (registered trade mark)) isinserted between the high-dielectric-constant dielectric 503 and thehousing 501. The high-dielectric-constant dielectric 503 is surroundedand held by this low-dielectric-constant material.

The high-dielectric-constant dielectric 503 of the embodiment isseparated into two sections and a slit 505 is formed in the spacingbetween the two sections in order to enhance the electric field insidethe resonator. Furthermore, the housing 501 is provided with a slit 504at a position opposite to the slit 505 of the high-dielectric-constantdielectric 503 in order to radiate electromagnetic waves.

As shown in FIG. 6, the high-dielectric-constant dielectric 503 of thedielectric resonator 304 is electromagnetically coupled to micro striplines 502 a and 502 b. FIG. 6 is a perspective view showing theconverter of the embodiment shown in FIG. 3. As can be seen from FIG. 6,one end of each of the micro strip line 502 a and 502 b is respectivelyformed on each of alumina substrates 602 a and 602 b and is connectedrespectively with a branch line of the electrodes 303 a and 303 bthrough bonding wires 605 a and 605 b.

An electrical signal induced at the electrodes 303 a and 303 b when anoptical signal propagates through the optical waveguide is transmittedto the micro strip lines 502 a and 503 a through the bonding wires 605 aand 605 b shown in FIG. 6 and is guided to the inside of the dielectricresonator 304. The various parameters of the dielectric resonator 304are designed such that this electrical signal resonates inside thedielectric resonator 304.

In order to accumulate efficiently the energy of the electrical signalusing the resonance inside the dielectric resonator 304, it ispreferable to provide a switch for selectively open and close the slit505, to the dielectric resonator 304. Because leak of theelectromagnetic field to the exterior is suppressed and the Q value ofthe resonator is increased while this switch is closed, the energy ofthe electrical signal is amplified. The electrical signal having theenergy amplified in the resonator is radiated to the exterior of theresonator as an electromagnetic wave by opening the switch.

The dielectric constant becomes drastically low relative to the insideof the resonator at the slit 505 while the slit 505 is open. Therefore,the electromagnetic field of the electrical signal propagated throughthe high-dielectric-constant dielectric 503 becomes drastically enhancedat the portion at the slit 505. Because the slit 504 of the housing 501is arranged in the vicinity of the portion where the electric field isenhanced in this manner, the electrical signal propagated through themicro strip lines 502 a and 502 b is converted into an electromagneticwave and is radiated from the slit 504 to the exterior of the resonator.

The dielectric resonator 304 in the embodiment is designed to resonatein TM₁₁δ mode. For example, when an electrical signal at 26 GHz isradiated as an electromagnetic wave, the length of the slit 504 is setat approximately 3 mm and the width of the slit 504 is set at 0.6 mm.

Assuming the dielectric constant of vacuum ∈₀ is 24 and the relativedielectric constant at the slit 504 ∈_(r) is 1, the electric fluxdensity D is equal to ∈_(r) ∈₀E and is constant both inside thedielectric resonator 304 and in the portion at the slit 504. Therefore,the electric field E is enhanced to a magnitude 24 times as strong asthe original magnitude when the electric field goes out of the inside ofthe resonator having a high relative dielectric constant to the exteriorof the resonator through the slit 504. Because the electric field energyis accumulated inside the dielectric resonator 304, the generatedelectric field energy itself can be enhanced by the resonator. In theembodiment, the Q value of the dielectric resonator 304 can be set ataround 2,000.

When the substrate 301 is formed from a DAST crystal, the electric fieldgenerated between the electrodes when an optical signal propagatesthrough the optical waveguide is around 80 μV/m. However, the electricfield can be enhanced to a magnitude exceeding 2,000 times as strong asthe original magnitude by using the resonator of the embodiment.

The high-dielectric-constant dielectric 503 is formed from, for example,a MgYiO₃—CaTiO₃-based ceramic. The cross section of thehigh-dielectric-constant dielectric 503 is 1 mm×1 mm and the length inthe longitudinal direction of the high-dielectric-constant dielectric503 is 5 mm.

The housing 501 in the embodiment has a shape of, for example, arectangle having a cross section of 3 mm×3 mm and the length in thelongitudinal direction of 15 mm. The housing 501 has a structure inwhich PTFE is filled with between the housing 501 and thehigh-dielectric-constant dielectric 503.

Dielectric ceramic materials represented by Zr—TiO₄.BaTiO₃ may be usedfor the high-dielectric-constant dielectric 503. Because the dielectricconstant differs according to the dielectric material, the dimensions ofthe housing 501 and the dimensions of the dielectric resonator 304 needto be changed.

The resonator in the embodiment is designed to resonate in TM₁₁ δ mode.However, the resonator may be designed to resonate in another mode suchas TE₁₀ mode. Furthermore, in the embodiment, though the slit 504working as the electromagnetic wave radiation mechanism is provided tothe housing 501 of the resonator 304, the amount of electric waveradiation can be increased by providing a conductive electrode in astate where the electrode is not grounded to the resonator housing. Theslit 504 provided to the housing 501 of the resonator 304 operates as aslot antenna. However, the slit 504 may be operated as a dielectricantenna by further mounting another dielectric resonator to the slit504.

As described above, according to the optical signal to electrical signalconverter of each of the above embodiments, an optical signal can beconverted into an electrical signal by inputting the optical signal intoan optical waveguide and propagating the optical signal through theoptical waveguide. Then, the electrical signal is amplified by theresonator. Therefore, an ultra-high-speed-modulated light beam can beaccurately detected.

Furthermore, the converter can be made compact by providing the opticalwaveguide, the electrodes and the antenna integrated on the basematerial. Therefore, when information is transmitted and receivedbetween various communication devices or between electric devices, it iseasy to incorporate the converter into any of those devices.

As described above, according to the embodiments of the presentinvention, it is also easy to control communication devices and electricdevices, by each other by radio, that can easily convert an opticalsignal sent through optical signal transmission means such as an opticalfiber, into a radio signal. Furthermore, dissemination of “net homeappliances” that are the existing home appliances controlled by radiocan be further facilitated.

According to the above embodiments, an optical signal is detected usingthe optical waveguide. Therefore, the sensitivity and the conversionefficiency can be enhanced due to the pseudo velocity matching.Especially, when the periodic polarization inversion structure is used,the converter can be applied to the detection of an optical signalhaving an arbitrary wavelength and at an arbitrary frequency by properlydesigning the polarization inversion period, the optical waveguide andthe dielectric resonator.

Needless to say, the various materials and the device compositions usedin the present invention are not limited to the materials andcompositions used in the embodiments described above. The optical signalto electrical signal converter of the present invention can be realizedusing materials other than the above-described dielectric materials andthe nonlinear optical materials.

INDUSTRIAL APPLICABILITY

According to the present invention, high-speed responses can be achievedbecause an optical signal can be converted into an electrical signalwithout utilizing the drift of electric charges (carriers). Furthermore,a high-efficiency conversion from an optical signal to an electricalsignal is realized by amplifying with a resonator an electrical signalconverted from an optical signal and radiating the electrical signalfrom an antenna as an electromagnetic wave. According to the presentinvention, an optical signal to electrical signal converter that issmall-sized and capable of high-speed operation can be provided.

1. An optical signal to electrical signal converter comprising: anoptical waveguide for receiving and propagating a modulated opticalsignal; and a pair of electrodes disposed within a region where anelectric field applies, said electric field being generated in theoptical waveguide by a nonlinear optical effect when the optical signalpropagates through the optical waveguide wherein the optical waveguideis formed on a dielectric substrate or in the dielectric substrate, andwherein the pair of electrodes are provided on a top surface of thedielectric substrate, said pair of electrodes being opposite to eachother sandwiching the optical waveguide, thereby detecting changes ofthe electric field.
 2. An optical signal to electrical signal converteraccording to claim 1, further comprising a resonator coupled to the pairof electrodes, the resonator being configured to be excited by anelectrical signal induced at the pair of electrodes by the electricfield.
 3. An optical signal to electrical signal converter according toclaim 1, wherein the optical signal comprises a side band signalcorresponding to a modulation frequency f_(m).
 4. (canceled)
 5. Anoptical signal to electrical signal converter according to claim 1,wherein at least a portion of the optical waveguide and at least aportion of the dielectric substrate are formed from a nonlinear opticalmaterial, and the electric field is generated by a differentialfrequency generation when the optical signal propagates through theoptical waveguide.
 6. An optical signal to electrical signal converteraccording to claim 5, further comprising an electromagnetic waveradiating device coupled to the resonator, wherein the optical signal toelectrical signal converter radiates the electrical signal as a radiosignal.
 7. An optical signal to electrical signal converter according toclaim 1, wherein the resonator and the electromagnetic radiating deviceare integrated with the substrate.
 8. An optical signal to electricalsignal converter according to claim 7, wherein the resonator and theelectrodes are connected by micro strip lines formed on the dielectricsubstrate.
 9. An optical signal to electrical signal converter accordingto claim 1, wherein the modulation frequency of the optical signal is 10GHz or higher.
 10. An optical signal to electrical signal converteraccording to claim 1, further comprising a light beam input portioncoupled to the optical waveguide.
 11. An optical signal to electricalsignal converter according to claim 5, wherein the nonlinear opticalmaterial is a material selected from a group consisting of lithiumniobate (LiNbO₃), lithium tantalate (LiTaO₃)-based material, potassiumtitanyl phosphate (KTiOPO₄)-based material, rare earth-calcium oxyborate(RECa₄O(BO₃)₃, RE: a Rare Earth element)-based material, DAST(4-dimethylamino-N-methyl-4-stilbazorium-toxyrate) and 3RDCVXY(dicyanovinyl termination-dimethyl substitution-diazo).
 12. An opticalsignal to electrical signal converter according to claim 1, wherein theoptical waveguide has a periodic polarization inversion structure wherethe polarization direction is inverted periodically along the opticalwaveguide.
 13. An optical signal to electrical signal converteraccording to claim 1, further comprising a resistor connectingelectrically the pair of electrodes with each other.
 14. An opticalsignal to electrical signal converter according to claim 1, furthercomprising a housing accommodating the dielectric substrate.